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Nitrate‐N uptake from soil depends on root growth and uptake activity. However, under field conditions N‐uptake activity is difficult to estimate from soil‐N depletion due to different loss pathways. We modified the current mesh‐bag method to estimate nitrate‐N‐uptake activity and root growth of two oilseed‐rape cultivars differing in N‐uptake efficiency. N‐efficient cultivar (cv.) ‘Apex' and N‐inefficient cv. ‘Capitol' were grown in a field experiment on a silty clayey gleyic fluvisol near Göttingen, northern Germany, and fertilized with 0 (N0) and 227 (N227) kg N ha–1.In February 2002, PVC tubes with a diameter of 50 mm were installed between plant rows at 0–0.3 and 0–0.6 m soil depth with an angle of 45°. At the beginning of shooting, beginning of flowering, and at seed filling, the PVC tubes were substituted by PVC tubes (compartments) of the same diameter, but with an open window at the upper side either at a soil depth of 0–0.3 or 0.3–0.6 m allowing roots to grow into the tubes. Anion‐exchange resin at the bottom of the compartment allowed estimation of nitrate leaching. The compartments were then filled with root‐free soil which was amended with or without 90 mg N (kg soil)–1. The newly developed roots and nitrate‐N depletion were estimated in the compartments after the installing period (21 d at shooting stage and 16 d both at flowering and grain‐filling stages). Nitrate‐N depletion was estimated from the difference between NO$ _3^- $‐N contents of compartments containing roots and control compartments (windows closed with a membrane) containing no roots. The amount of nitrate leached from the compartments was quantified from the resin and has been taken into consideration in the calculation of the N depletion. The amount of N depleted from the compartments significantly correlated with root‐length density.Suboptimal N application to the crop reduced total biomass and seed‐yield formation substantially (24% and 38% for ‘Apex’ and ‘Capitol’, respectively). At the shooting stage, there were no differences in root production and N depletion from the compartments by the two cultivars between N0 and N227. But at flowering and seed‐filling stages, higher root production and accordingly higher N depletion was observed at N0 compared to N227. Towards later growth stages, the newly developed roots were characterized by a reduction of root diameter and a shift towards the deeper soil layer (0.3–0.6m). At low but not at high N supply, the N‐efficient cv. ‘Apex’ exhibited higher root growth and accordingly depleted nitrate‐N more effectively than the N‐inefficient cv. ‘Capitol’, especially during the reproductive growth phase. The calculated nitrate‐N‐uptake rate per unit root length was maximal at flowering (for the low N supply) but showed no difference between the two cultivars. This indicated that the higher N‐uptake efficiency of cv. ‘Apex’ was due to higher root growth rather than higher uptake per unit of root length.

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A 3-year field experiment was carried out to determine the significance of root-growth characteristics contributing to N-uptake efficiency of two oilseed rape (Brassica napus L.) cultivars differing in N efficiency. Two N treatments were applied, and the core and minirhizotron techniques were used to study root-length density and number of living roots, respectively. Fertilizer-N supply increased shoot dry matter, grain yield, total N uptake, and total soil N min contents particularly in the top soil. Although significant differences occurred in all parameters between years, the interactions between years and cultivars were mostly not significant. Compared to cv. Capitol, the N-efficient cv. Apex was characterized by a higher grain yield at N0 and a higher N uptake during reproductive growth. This genotype also had a higher root-length density and more living fine roots particularly in the topsoil layer. Root growth of this genotype was especially high from beginning of shooting to beginning of flowering, while shoot growth and N uptake during vegetative growth were comparatively low. Our results suggest that N-efficient cultivars can be characterized by a high investment in root growth during the vegetative stage with a comparatively slow shoot growth and N-uptake rate until beginning of flowering, which, however, continues during reproductive growth. High root production only during reproductive growth seems to be less effective to achieve high N efficiency, because this may lead to a shortage of assimilates for seed filling. High root-length density at vegetative stages may thus be advantageous for N uptake and reproductive growth and could be a useful morphological character for the selection and breeding of N-efficient cultivars.

Reuse of agricultural waste materials is a smart solution for reducing their environmental impacts and increase economic value. Spent coffee grounds (SCG) is worldwide generated in tremendous amounts. The objectives of this study were to optimize a method to extract humic-like substances (HLS) from SCG using KOH extractant, prepare a liquid organic-mineral fertilizer enriched with N and P in addition to K, and to evaluate this fertilizer on growing maize (Zea mays L.). HLS extracted from SCG increased with KOH concentration up to 3 N and with extraction ratio up to 1:10 at room temperature. Increasing the temperature to 50 and 80 °C and contact time up to 3 h, significantly enhanced HLS recovery. Therefore, the optimum conditions for maximum HLS extraction were; 2 N KOH, SCG-to-extractant ratio of 1:10, 3 h of contact time, and 80 °C. Adjusting the pH of the alkaline K-HLS supernatant to pH 6 was achieved using a mixture of HNO 3 and H 3 PO 4. The prepared NPK mineral-organic fertilizer (NPK-HLS) was dark brown containing 2.0/5.8/8.6 as N/P 2 O 5 /K 2 O and 5.1 %w/v HLS. The produced fertilizer was evaluated for its effect on maize plants grown in a calcareous soil low in available nutrient using a CRBD pot experiment. Two application rates of NPK-HLS (NPK-HLS1 and NPK-HLS2) based on P rates, 125 and 250 mg P pot-1 , with and without a basic fertilizer (BF) were applied. Both NPK-HLS1 and NPK-HLS2 rates significantly increased shoot dry matter, NPK uptake and availability in soil compared to the control, but did not significantly affect root dry matter. Increasing the application rate to NPK-HLS2 did not further increase dry matter or NPK uptake. No significant effect was found for BF for all measured variables. Results of this study showed that the extraction of HLS from SCG and its incorporation into a liquid fertilizer could be an alternative solution to the reuse of SCG.

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